The present invention relates generally to a method and apparatus for generating correction signals for use in forming low distortion analog signals and, more particularly, to a correction signal generator that encodes a desired signal waveform into a digital data signal and a digital correction signal and stores such signals, and subsequently reads out the stored digital signals and decodes the signals into analog signals that may be summed into the desired low or ultra-low distortion analog signal.
Recent advances in analog signal processing technology have created a need for testing and other devices capable of generating purer and purer analog signals. Digitizers (such as A/D converters) now exist that are capable of 120 dB of dynamic range without interference from noise or distortion, and in the near future it is anticipated that signal processing technology will exist for analyzing analog signals with signal to noise and distortion ratios as high as 150 dB. However, before the new signal processing technology can be effectively implemented, especially in field applications such as seismic exploration, a means must exist for calibrating and testing these new digitizers. Because there are additional constraints such as power and weight limitations affecting equipment used in the field, the solution to this need has been elusive until now.
It should be recognized that there are many devices used for generating analog signals which are well known in the art. In particular, it is well known that many varieties of D/A converters exist which can be used to convert digital signals to analog signals. Further, successive approximation D/A converters, particularly when oversampled, have been used with some success to achieve signal to noise and distortion ratios of up to 100 dB for digital signals having as many as 18 bits.
However, it is difficult to achieve greater than 100 dB signal to noise and distortion ratios using such technology. For example, the successive approximation converters typically use a weighting source, whether it be a current source, voltage source, or resistors. These weighting sources are difficult to control and maintain accurate over time due to temperature fluctuations and other variables, particularly when more than 16 to 18 bits of input are used in the D/A converter. Thus, the recent advances in analog signal processing technology render the past state of the art of test signal generation practically useless; a new means of generating analog signals with a signal to noise and distortion ratio exceeding that of the new A/D technology is needed. Further, there is a particular need for analog signal sources that are portable and can be used for field calibration of the new digitizers.
A solution to this problem might be found in the use of a device that converts a continuously generated digital datastream into a desired analog signal. Such a datastream might be a sequence of discrete time data values, such as a digital bitstream. A device such as a DPCM (differential pulse code modulator) might then be used to convert, or used as a model for a program to computationally convert, the datastream into an analog signal. Also, there might be advantages in using a computational program modeled on one bit D/A converter circuits, found in the feedback loops of sigma delta digitizers. Such one bit D/A converters provide linear signals and, particularly with the well-known sigma delta converter, make encoder/decoder modeling easier.
But, while such an approach may produce low noise signals, the approach would not be feasible for applications such as field use. The computations required to continuously generate an appropriate digital bitstream, capable of conversion to a desired low or ultra-low distortion analog signal, would include high speed reiterative solutions of a system of equations and require a continuous high level of power consumption. This level of computational power is highly undesirable in field equipment, which is desired to operate at as low power as practical. In fact, with respect to field applications it is estimated that the equipment needed for computing such a signal could require as much as double the power needed for current field testing equipment, and would increase the speed and cost of the computational equipment.
Thus, there is a continuing need for a system of generating a digital signal and using this signal to form an analog signal with low or ultra-low distortion, but without significant extra requirements or expenses for computational equipment and power supplies.